Refineries use large quantities of hydrogen in daily operations. Hydrogen is currently produced mainly via steam methane reforming (SMR). SMR has associated CO2 emissions that has been identified as an area for potential reduction of greenhouse gas emissions within refineries.
Technologies such as amine scrubbing can remove CO2 from process gas streams, such as the gaseous waste stream from a SMR unit. Heat for the endothermic process is typically provided by combustion of natural gas and/or high BTU process streams (for example fuel gas or pressure swing absorber offgas) using air as the oxidant. Because the natural gas is burned in the presence of air, the exiting flue gas comprises mostly nitrogen. While the concentration of CO2 in the flue gas is relatively low, the flow rate is large, such that approximately 50% of the CO2 produced during SMR is emitted during combustion. The balance of the CO2 comes from the reforming reaction itself and is produced in a stream with a relatively high CO2 partial pressure (typically around 45 psia), such that the CO2 can be effectively captured using amine scrubbing technology. On the other hand, the combustion flue gas stream is mostly nitrogen, thus it is cost prohibitive to size an amine scrubber to process the entire stream. The result is an upper limit to how much CO2 could be captured practically from a conventional SMR unit. This could have a significant impact on the economics of a SMR unit if CO2 emission taxes, incentives, and/or caps were imposed.
One way that others have proposed to solve this problem is by using an oxygen-fuel combustion process within the SMR furnace. In oxygen-fuel combustion process, the furnace fuel is combusted in the presence an oxygenated environment rather than air. The resulting flue gas primarily consists of CO2 and H2O with very little nitrogen. The H2O in the flue gas is easily removed through condensation, leaving a stream of concentrated CO2 that is ready for compression, transportation and sequestration. This process greatly increases the amount of CO2 that could be captured practically from a SMR plant.
One of the largest obstacles in operating an oxygen-fuel combustion process within the SMR boiler furnace is obtaining oxygen. Typically, the only proposed method of obtaining oxygen to an SMR plant is the use of an air separation unit. The problems with air separation units is that they typically separate oxygen and nitrogen from air through a cryogenic separation process, which has large capital, operating costs and CO2 footprint associated.
There exists a need to produce oxygen in high quantities and purity in a safe and cost effective manner to operate with an oxygen-fuel combustion process within a SMR furnace.